The present invention relates to the growth of bulk semiconductor materials in a manner which provides a possibility to manufacture bulk crystals in the form of ingots, fabricate substrates from these ingots and thus enhancing the resulting performance of devices made from those semiconductors. In particular, the invention relates to the method of growing gallium nitride (GaN) ingots and epitaxial layers from the melt-solutions.
Resent results in fabrication of GaN-based light-emitting diodes (LEDs) and laser diodes (LDs) operating in green, blue, and ultra violet spectrum region have demonstrated tremendous commercial potential of nitride semiconductors. Because of lack of GaN substrates, these devices have been developed on the sapphire or silicon carbide substrates and are suffering from high defect density in the device structures including high density of threading dislocations, up to 1010 cmxe2x88x922, domains and grain boundaries. Destructive influence of these imperfections on the device performance has been demonstrated in a number of publications. Recently, in S. Nakamura, M. Senoh, S. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyouku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, Applied Physics Letters, Vol. 72, p. 2014 (1998), the fabrication of LDs on free-standing reduced-defect-density 80 xcexcm thick GaN substrate grown by hydride vapor phase epitaxy with lifetime longer than 780 hr. and threshold current density of 7 kA/cm2 was reported. In contrast, the LDs fabricated under similar conditions but on a sapphire substrate exhibited shorter lifetime of 200 hr at lower operating current density.
The fact that misfit dislocations, grain boundaries, domains and residual stresses greatly reduce performance of GaN-based devices and cut their applications was experimentally proven. The main issue in GaN-based technology is lack of GaN and AlN substrates.
Foreign substrates including Al2O3, SiC, ZnO, LiGaO2, LiAlO2, and ScMgAlO4 have been tested for GaN heteroepitaxial growth. Lattice and thermal mismatch between foreign substrate and grown GaN-based device structure originate most of the defects. It is clear that only GaN substrates will allow one to reduce defect density in GaN devices and improve device characteristics.
The main challenge in growing GaN substrates is incongruent decomposition of GaN material by sublimation that becomes noticeable at temperatures from 800-1100xc2x0 C. A number of attempts to realize growth of bulk and quasi-bulk GaN crystals from vapor phase have been done. Natural ways to overcome the decomposition problem are (1) to use the chemical transport technique or (2) sublimation growth at high pressure. Both methods have been applied to grow GaN layers but due to technological difficulties no GaN ingots were grown. In these methods, thick GaN layers were grown on foreign substrates and had high defect density.
Another method to grow GaN crystals is the growth from liquid phase. The main problem in liquid phase growth of GaN from liquid phase is extremely low solubility of nitrogen in melts, particularly in Ga melt. GaN crystals having area up to 200 mm2 and thickness up to 0.2 mm were grown by melt-solution technique (S. Porowski, Proceedings of the Second International Conference on Nitride Semiconductors ICNS""97, Tokushima, Japan, Oct. 27-31, 1997, p. 430). These GaN crystals were spontaneously nucleated and grown from nitrogen-gallium melt-solution. In order to overcome low nitrogen solubility problem, growth temperatures from 1500-1600xc2x0 C. and nitrogen gas pressure from 10-20 kbar are required to grow GaN crystals. Even at these high pressures and temperatures nitrogen solubility in gallium melt is very low. As a result, at 20,000 bar and 1500xc2x0 C. growth rate of about 0.01-0.05 mm/hr can be obtained. Lateral growth rate (growth rate perpendicular to [0001] crystallographic direction) was about 1 mm/day. Undoped GaN crystals grown by this method have high background electron concentration and did not exhibit edge luminescence under optical excitation. GaN ingots were not grown by this technique.
Another attempt to grow GaN crystal from Gaxe2x80x94N melt-solution was undertaken by Takayuki Inoue, Yoji Seki, Osamu Oda, Satoshi Kurai, Yoichi Yamada and Tsunemasa Taguchi, Jpn. J. Appl. Phys. Vol. 39 (2000) pp. 2394-2398. GaN crystals up to 10 mm in diameter were grown at 1475xc2x0 C. under a nitrogen pressure of 0.98 GPa. High pressure in combination with high temperature required for both above methods make it difficult to perform controllable GaN crystal growth using GaN seed and develop these methods as production techniques.
One way to increase nitrogen solubility is to use not pure Ga melt but Ga with some additives. Alternative melts were used in D. Elwell, R. S. Feigelson, M. M. Simkins, and W. A. Tiller, Journal of Crystal Growth, Vol. 66, p. 45 (1984). Growth was carried out in the temperature range from 900 and 1000xc2x0 C. A sapphire wafer used as substrate was placed in either end of the furnace and the boat was charged with 50 g of 99.9999% pure gallium, Ga/Bi and Ga/Sn alloys. Ammonia gas served as nitrogen source. Ammonia partial pressure was (1.5-2)xc3x9710xe2x88x923 bar. As carrier gas hydrogen or argon were used. In some experiments, GaN seeds were employed. The growth reaction proceeded for 10 days. The GaN deposition was in the form of small crystallites randomly oriented with respect to the seed crystals. The largest crystal grown, of 2.5 mm in length, was part of a cluster of three crystals grown at 930xc2x0 C. on SiC plate with ammonia partial pressure of 1.08xc3x9710xe2x88x923 bar. The use of seed crystals appeared to have no beneficial effect on crystal size. The addition of Bi to the solution was found to increase the number of crystallites nucleated. Tin was tried as an alternative solvent component. The major advantage of tin is that it reacts with nitrogen giving atomic nitrogen in solution. It was therefore considered possible that the solubility of atomic nitrogen in molten Ga/Sn alloy would be higher than that in Ga melt. Alloys with 10-80 at. % content of Sn were tested. Nitrogen gas was used in place of the NH3+H2 mixture with a slow growth rate of about 150 cm3/day. Some GaN growth was observed, together with oxide impurities. But, in all these experiments the crystallites were smaller than pure gallium was used. GaN ingots were not grown by this technique.
Alternative way to introduce nitrogen in Ga melt to grow polycrystalline GaN was described in A. Argoitia, C. C. Hayman, J. C. Angus, L. Wang, J. S. Dyck, and K. Kash, Applied Physics Letters, Vol. 70, p. 179 (1997). Plasma gun was used to increase the thermodynamic activity of the nitrogen in order to raise the nitrogen concentration in the gallium. The active species in the plasma include N, N+, N2+, and excited states of N2. Recombination of N to form N2 is strongly favored thermodynamically, however, this recombination is sufficiently slow within the gallium melt to permit the parallel formation of GaN. Synthesis of GaN was achieved by directing plasma from electron cyclotron resonance microwave source (ECR-source) onto a liquid Ga pool heated of up to 1000xc2x0 C. in BN crucible. The ECR source was mounted directly above the crucible and gave an ion flux density of 1016 cmxe2x88x922secxe2x88x921. The partial pressure of atomic nitrogen in the beam is approximately 0.05 mTorr. An argon plasma was employed for 10 min followed by a hydrogen plasma for 30 min to clean melt surface. The hydrogen flow was replaced by 10 sccm of nitrogen and the temperature raised slowly to 1000xc2x0 C. During this step, the pressure was fixed at 0.5 mTorr. After 15 min., at a temperature of 700xc2x0 C., the growth of a crust of polycrystalline GaN began on the melt surface. The nitrogen plasma was maintained for 12 hr at the final temperature of 1000xc2x0 C. At the end of a run, a polycrystalline GaN xe2x80x9cdomexe2x80x9d completely covered the Ga melt. A typical xe2x80x9cdomexe2x80x9d was 0.1 mm thick, weighed 40 mg, and had an surface area of 70 mm2. The average linear growth rate was about of 8 xcexcm/hr. GaN crystalline ingots were not grown by this technique. Seeded growth technique was not applied. Plasma source located above the melt make difficult to introduce a seed in the melt.
Recently, in L. Allers, R. Clampitt, J. N. Hiscock, and A. T. Collins, will be published in Proc. Intern. Conf. SiC, III-nitrides and Related Mater., Stockholm, Aug. 31-Sep. 5, 1997, the growth of 2xc3x972xc3x970.1 mm3 2 Hxe2x80x94GaN single crystals on the surface of heated Ga melt over the temperature range of 700-1100xc2x0 C. using commercially available high density N-atom plasma source was reported. The method of synthesis was similar to that of described by Argoitia et al. in the previous section. The growth was undertaken in an unbaked diffusion pumped system with base pressure 10xe2x88x926 Torr. A sample of Ga/Al/In was heated to 600-1200xc2x0 C. (depending on the particular metal) in a flat BN boat under N2 flow for 10 min. An N atom source was mounted in close proximity to the melt and allowed to irradiate it with atomic nitrogen over a period of 3-4 hr., during which more than 100 xcexcm nitride layer was formed on its surface. An average linear growth rate was found to be 12 xcexcm/hr, but in some surface areas it exceeded 25 xcexcm/hr. The convex surface of the xe2x80x9cdomexe2x80x9d was relatively featureless and no distinct crystalline facets could be detected. In contrast, the concave interior of the xe2x80x9cdomexe2x80x9d was predominantly covered with relatively large crystallites. These were mostly hexagonal randomly oriented and with diameters ranging from 10 to 100 xcexcm. There were a number of single crystals up to 3 mm in diameter and 100 xcexcm in thickness.
In all techniques mentioned above, there were no gallium nitride ingots grown. with or without seeds and a seed was not used to enlarge GaN crystal dimensions.
Thus, although GaN offers tremendous potential for optoelectronics and high-power high-frequency devices, such devices will realize their full potential only when crystal growth method to fabricate large size GaN crystalline ingots willbe developed.
It is an object of the present invention to provide a new advanced method forproducing a crystalline compound material, including large diameter GaN crystals.
Another object of the invention is to significantly reduce cost of the growth equipment for producing GaN crystals.
Another object of the invention is to simplify the growth process for producing GaN crystals.
Another object is to increase growth rate of GaN crystals.
Another object is to increase size of GaN crystals.
The invention meets these objects with a method of producing GaN from liquid or vapor phase at low temperature and ambient pressure comprising a step of applying electric field to produce GaN crystalline material. For the invented method, high temperature and high pressure equipment is not required.
Growth of GaN takes place at temperature less than 1100xc2x0 C. and ambient pressure less than 2 atm. Using the electric field allows one to grow GaN crystals having 2 inch and larger diameter with growth rate, which can reach and exceed the value of 1 mm/hr.
GaN crystalline ingots can be grown on seeds and size of the grown GaN crystals can exceed the size of the seed in a few times.
GaN crystalline ingots can be grown on seed by drawing the growing crystal from or inside the melt.
GaN crystalline ingots can be nucleated on melt surface without seed.
These and other objects are further understood from the following detailed description of particular embodiments of the invention. It is understood, however, that the invention is capable of extended application beyond the precise details of these embodiments. Changes and modifications can be made to the embodiments neither affect the spirit of the invention neither exceed its scope, as described in the appended claims. The objects, advantages and features of the invention are illustrated by the accompanying drawings, wherein: